Method of fabricating directly heated cathode



Gd 8, 1968 s. E. AUNGST ET AL 3,404,442

METHOD OF FABRICATING DIRECTLY HEATED CATHODE Filed April 20, 1966 Ebazerfl- 55 c I Q: 2'?- Si F l i jz M .36 j Z4 2 a 1, Z 66 INVENTORS Srwuy 5 404/657" United States Patent 3,404,442 METHOD OF FABRICATING DIRECTLY HEATED CATHODE Stanley E. Aungst, Scottsdale, Ariz., and Robert A. Lee,

Lancaster, Pa., assignors to Radio Corporation of America, a corporation of Delaware Filed Apr. 20, 1966, Ser. No. 543,961 6 Claims. (Cl. 29-25.14)

This invention relates to a method of fabricating directly heated cathodes, and particularly to a method of fabricating filamentary cathodes of the basket weave type having fast warm-up times.

A basket weave cathode comprise a hollow cylinder formed from a woven mesh or screen of fine wire, the cylinder being mounted at its ends between a pair of spaced apart members. As known, the finer the wire used in the wire mesh, the faster is the warm-up time of such cathodes. A problem associated with the use of very fine wires, however, is that the mesh cylinders formed therefrom have little structural strength, whereby the fabrication of cathodes is a delicate, ditficult, and expensive process.

An object of this invention is to provide a novel and improved method of fabricating filamentary cathodes, and particularly cathodes of the basket weave type.

A further object of this invention is to provide a novel and improved method of fabricating basket weave cathodes having exceptionally fast warm-up times, and having greater strength than prior known cathodes of this type.

For achieving these objects, two support for the mesh cylinder are mounted in spaced apart relation on a mandrel of a disposable material, such as a relatively low temperature melting plastic. A wire mesh is then formed around the mandrel between the supports and on portions of the supports. The ends of the mesh are then bonded to the supports and the wire strands of the mesh are bonded to one another at their cross-over points. A spacing member is bonded to the two supports, and the mandrel disposed of, as by melting. Subsequent operations include the coating of electron emissive materials onto the mesh strands and the mounting of the cathode assembly into an electron tube.

In the drawings:

FIG. 1 is an elevation, in section, of a cathode assembly;

FIG. 2 is an elevation, in section, of a mandrel assembly;

FIG. 3 is an exploded view, in section, showing two subassemblies used in the fabrication of the cathode assembly shown in FIG. 1; and

FIG. 4 is an elevation, in section, showing the two subassemblies shown in FIG. 3 in assembled relation.

The cathode assembly shown in FIG. 1 comprises a tubular, central spacing member 12, and an annular top cap 14 secured to the upper end of the spacing member 12. The top cap 14 has an annular peripheral flange 16, and the spacing member 12 has a top cap locating embossment 18. Although not shown, the planar portion of the top cap 14 may be slotted to accommodate thermal expansions and contractions of the cathode assembly during on-off cycling operation thereof. Mounted on the spacing member 12 adjacent to its lower end is a terminal assembly 22 including a washer 24 secured to the mem ber 12, a tubular insulating spacer 28, and a cup-shaped member 30. A connector strap 32 is secured to the lower end of the spacing member 12. Each of the aforementioned members, with the exception of the spacer 28, is made from an electrical conducting material such as "ice nickel. The spacer 28 is a ceramic material such as alumina.

Coaxially mounted on the cup-shaped member 30 is a tubular heat dam and filamentary support 36 of an electrically conductive material, such as nickel, having a thick wall end section 38 and a thin wall section 40. Mounted between the top cap 14 and the heat darn 36 is a cylindrical wire mesh filament 44 of a refractory material, such as 1 mil diameter tungsten wire. Other known filament wires such as nickel, nickel alloys, molybdenum, or the like, can be used for the filament 44. The filament 44 is coated with an electron emissive material such as a mixture of barium carbonate and strontium carbonate which are subsequently converted to barium oxide and strontium oxide.

The thin wall section 40 of the heat dam 36 is provided to reduce heat loss from the filament by conduction through the heat dam 36. Although not shown, the upper wall section 39 of the heat dam on which the filament 44 is mounted is preferably thickened for reasons of greater strength.

In the operation of the cathode assembly 10, an energizing voltage is applied between the connector strap 32 and the cup-shaped member 30. Cathode assemblies of the type shown herein are known and are being presently commercially used in tube types such as RCA 8462. For this reason, details of the means for mounting the cathode assembly 10 in electron tubes and details of the electron tubes are not presented.

In the fabrication of the cathode assembly 10, the top cap 14 and the heat dam 36 are mounted on the ends of a tubular mandrel 48, as shown in FIG. 2. Preferably, a stepped mandrel is used to provide a mandrel assembly 50 having a constant outer diameter along the length of the mandrel. The mandrel 48 is made of a disposable material. In one embodiment, the mandrel 48 is made of a rigid but low temperature melting material, such as known plastic materials such as polystyrene, methyl methacrylate, or the like. The selection of other suitable materials, if desired, will be apparent to those skilled in the art.

The wire mesh filament 44 is then formed around the mandrel 48 and along the flange 16 of the top cap 14 and along the upper wall section 39 of the heat dam 36.

To accomplish this, according to a preferred method (but not illustrated), the mandrel assembly 50 is mounted on a solid mandrel mounted, in turn, on a lathe. The end of a filament wire fed from a wire feed is secured to the mandrel assembly, as by cement, and the wire feed is caused to reciprocate axially along the length of the mandrel 48 while the mandrel assembly is rotated about its longitudinal axis. The various mandrel and apparatus members referred to are not shown since such members are known to persons skilled in the art. Also, details of the resulting mesh filament, such as the mesh configuration, the turns per inch, or the like, are not given since such mesh filaments are presently known and used.

The mandrel assembly 50, now including the filament 44, as shown in the upper portion of FIG. 3, is removed from the winding apparatus and further processed to bond the ends of the filament 44 to the flange 16 of the top cap 14 and to the wall section 39 of the heat dam 36, and to bond the mesh strands to one another at the strand crossover points.

One means to accomplish this is to coat the filament 44, the top cap 14, and the heat dam 36, as by known electroplating or electroless plating processes, with a metal such as nickel. A coating of a thickness of 0.2 mil or less is found satisfactory. The mandrel 48 is made from an insulating material, such as the aforedescribed plastic material, hence is not plated. The plating mechanically bonds the filament strands to one another and the filament 44 3 to the top cap 14 and to the heat dam 36. The nickel plating provides the cylindrical filament 44 with sufficient strength and stiffness to be rigidly selfsupporting upon subsequent removal of the mandrel 48. The bonding of the filament strands to one another is an important feature with respect to permitting the use of fine filament wire, such as one mil tungsten, and providing a high production yield.

For strengthening the nickel plated joints, the assembly 50 may be heated to cause the nickel to self-diffuse and form nickel-to-nickel diffusion bonds. In the instance wherein a plastic mandrel 48 is used, the heating can be done as part of the mandrel removal step, to be described.

A minimum thickness of nickel, e.g., in the order of 0.1 mil thickness, is generally preferred. One reason is that the thinner the nickel coating, the faster is the cathode warm-up time. A further reason is that a thin coating of nickel is found to minimize cross-diffusion of tungsten from the filament wires with the nickel coating. In some instances, it is found that the tungsten-nickel diffusion causes embrittlement and weakening of the tungsten wires during the operating life of the cathode in an electron tube. To prevent such tungsten-nickel diffusion, a diffusion barrier of a material, such as rhenium, can be provided between the tungsten filament wires and the nickel plating. The rhenium can be flash coated onto the tungsten wire prior to the winding of the Wire onto the mandrel assembly 50. Nickel does not diffuse through rhenium and rhenium does not react with tungsten.

Instead of the nickel plating, the bonding of the mandrel assembly 50 may be accomplished with coatings of other metals not incompatible with the emission of electrons from cathodes, such as platinum or chromium. Platinum and chromium form a diffusion bond with tungsten while not causing embrittlement of the tungsten. However, for reasons of greatest strength with minimum plating thickness, important for minimizing the warm-up time of the cathode assembly, a nickel plating is generally preferred.

The remaining members of the cathode assembly 10 are now mounted onto the mandrel assembly 50. Prior to this, the spacing member 12 and the terminal assembly 22 have been preassem-bled as a subassembly 56 (as shown in the lower portion of FIG. 3) by disposing these members in proper relationship in a jig, not shown, and brazing the Washer 24 to the spacing member 12, the washer 24 to the insulating spacer 28, and the insulating spacer 28 to the cup-shaped member 30. The surfaces of the insulating spacer 28 brazed to other members are pre-metallized, as with molybdenum, using known processes. A

copper or silver copper eutectic brazing material can be used, for example, to braze the various members together.

The subassembly S6 is inserted into the mandrel assembly 50 through its lower end 60 and positioned therein (FIG. 4) with the embossment 18 of the spacing member 12 engaged with the top cap 14, and with the lower end 64 of the heat dam 36 extending slightly beyond the lower end 66 of the cup-shaped member 30. An annular shoulder 70 is thus provided at the lower end of the mandrel assembly 50.

The mandrel 48, if made from a relatively low temperature melting material, is eventually removed by a heating process. It is found preferable to restrain axial expansion of the mandrel 48 during the heating step to prevent axial stretching of the filament 44. Such stretching tends to weaken or break the joints at the cross-over points of the filament strands.

One means of restraining axial expansion of the mandrel 48 is to secure the su bassembly 56 to the top cap 14 and to the heat dam 36 prior to the mandrel melting step. A brazing ring, not shown, is placed on the annular shoulder 70 at the bottom of the cathode assembly 10. A localized heating source, such as a radio frequency coil, is used to heat and melt the brazing ring to secure the heat dam 36 to the member 30 of the suhassembly 4 56 while not heating the mandrel 48. The top cap 14 ma be secured to the top end of the spacing member 12 by mechanical means such as, for example, outwardly flaring the top end of the spacing member 12 with a conical flaring tool, not shown.

Alternately, after the brazing of the heat dam 36 to the subassembly 56, a simple clamp, not shown, may be used to press and hold thetop cap 14 againt the em bossment 18 of the spacing member 12.

The top cap 14 is then permanently bonded to the spacing member 12. This can be done at the same time as the disposal of the mandrel 48. Using a polystyrene mandrel 48, the cathode assembly is heated to a temperature of around 500 C., or if desired, to a temperature of around 750 C. for 30 minutes to cause the diffusion bonding of the nickel-to-nickel joints. The polystyrene melts and flows off the cathode assembly into the furnace from where it eventually vaporizes. During this process, the top cap 14 can be brazed to the top end of the spacing member 12 using a ring of brazing material, such as a silver-copper eutectic, previously disposed around the upper end of the spacing member 12.

In another embodiment, the mandrel 48 is made of an insulating material which is subject to chemical dissolution by a solvent not affecting the other portions of the cathode assembly. An example of such material is methyl methacrylate which can be dissolved by an etchant such as acetone.

By mounting the mandrel assembly 50 on the support ing sub-assembly 56 prior to removal of the mandrel 48, handling of the fragile filament mesh structure by itself, and the attendant possibility of damage thereto, is avoided.

The filament 44 is then provided with an electron emissive coating in the usual fashion, and the cathode assembly is thus completed and ready for assembly into an electron tube.

Electron tubes having cathodes of the type herein described having been fabricated having warm-up times in the order of A second or less. This is achieved by the use of extremely fine filament wire made practical by the described method. Electron tubes using cathodes of the type herein described but made according to prior art methods and using thicker filament wire have warm-up times in excess of one second.

What is claimed is:

1. A method of fabricating a cathode comprising:

mounting a pair of supports in spaced apart relation on a disposable mandrel,

forming a filamentary structure covering portions of said mandrel and portions of said supports,

bonding said filamentary structure to said supports,

securing a spacer member to said supports for maintaining said supports in spaced apart relation, and disposing of said mandrel.

2. A method of fabricating a cathode as in claim 1 wherein:

said filamentary structure is formed by winding a wire into a mesh onto said mandrel and onto portions of said supports, and

said structure is bonded by coating a metal onto said mesh and said supports, whereby the strands of said mesh are bonded to each other at their cross-over points, and said mesh is bonded to said supports.

3. A method of fabricating a cathode as in claim 2 wherein:

said supports are mounted on a heat disposable mandrel,

and

said mandrel is disposed of by heating said mandrel to a temperature above its melting point.

4. A method of fabricating a cathode as in claim 3 wherein:

said mandrel is made of an insulating material, and

nickel is plated onto said mesh and support.

5. A method of fabricating a cathode comprising:

mounting a pair of annular supports in spaced apart relation on a mandrel, said mandrel being of an insulating, heat disposable material,

winding a cylindrical wire mesh onto said mandrel and onto portions of said supports,

electroplating nickel onto said structure and said supports for bonding said supports to said structure and for bonding the strands of said mesh to each other at their cross-over points,

inserting a spacer member through said cylinder mesh in spaced relation therewith and into contact with said supports,

securing said spacer member to said supports, and

heating said mandrel for melting and disposing of it.

6. A method of fabricating a cathode as in claim 5 wherein:

said mesh is wound from tungsten wire, and

6 a coating of rheniurn is provided on said wire mesh prior to said electroplating step.

References Cited UNITED STATES PATENTS 3,197,390 7/1965 Mears 204l1 2,869,015 1/1959 Bolz 2925.14 1,946,603 2/1934 Von Wedel 20416 FOREIGN PATENTS 603,849 8/ 1960 Canada. 471,810 9/ 1937 Great Britain.

15 HOWARD S. WILLIAMS, Primary Examiner.

T. TUFARIELLO, Assistant Examiner. 

1. A METHOD OF FABRICATING A CATHODE COMPRISING MOUNTING A PAIR OF SUPPORTS IN SPACED APART RELATION ON A DISPOSABLE MANDREL, FORMING A FILAMENTARY STRUCTURE COVERING PORTIONS OF SAID MANDREL AND PORTIONS OF SAID SUPPORTS, BONDING SAID FILAMENTARY STRUCTURE TO SAID SUPPORTS, SECURING A SPACER MEMBER TO SAID SUPPORTS FOR MAINTAINING SAID SUPPORTS IN SPACED APART RELATION, AND DISPOSING OF SAID MANDREL. 